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Posted by George on Feb 11 2005
Forestar, Like I posted earlier in this thread, the bullet jump number on these high pressure rounds is pretty important. And, most serious reloaders here probably already know it but then again some may give it little thought. There are numerous sources out there that quote .010" (ten thousandths of an inch) clearance between bullet and lands for the best accuracy- without any caveats. BUT! experience has taught me to approach that with caution. Several cumulative reasons. One is that the very shallow engagement angle between the bullet ogive and the tapered land entry into the bore makes taking a precise bullet jam reading very difficult. I've tried most of the methods used and nearly always my readings vary as I try to repeat the measurement. Another measurement to watch carefully is the OAL based on base of case to bullet TIP, as it may vary greatly bullet to bullet and as it relates to the bullet jam number. Yet it is used a lot by reloaders. That number I think is listed in the manuals more for magazine clearance purposes than it is for correct or best bullet seating depth. Also, be sure to take (or verify) the bullet jam index measurement with a properly sized, headspaced case. I usually take the measurement three times and use the lowest (most conservative) number. Then set the bullet seating die for about a .025" to .035" bullet jump (.025"-.035" shorter than the jam index measurement) for a high pressure hunting load. [I've gone as low as .007" for a near max. target load- but not a hunting load] I then start working the load up with whichever powder/primer I've chosen, beginning with a reduced starting load. I know many of the benchrest guys start with a .010" jump number- but sticking a case in the chamber (or worse) because of a pressure spike out of the blue, during a very expensive hunt in the middle of no where is not a good thing. But, if the reloader feels comfortable with the measurements, pays attention to detail and the rifle shoots a whole lot better at .010" bullet jump- who am I to say one way or the other? Sorry about the lengthy post- just a few nuggets I've picked up over the years in striving for the best reliability and accuracy in my hunting loads/rifles. Good shooting!
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